As a bearing device for use in spindle motors of hard disks, polygon mirrors, optical disk devices or the like, a hydrodynamic bearing is being extensively used in place of a conventionally available ball bearing. As compared to the ball bearing, the hydrodynamic bearing is excellent in rotational precision and silentness. The demand for miniaturization and high rigidity of the spindle motors grows stronger because their use has been expanded to portable equipments in recent years.
Japanese Patent Laid-open Publication No. 2000-283154 (“prior art reference 1”) discloses a small-sized and high rigidity spindle motor arrangement wherein, as shown in FIG. 4, encircling annulus member 2 having radial dynamic pressure creating grooves 10 on its outer peripheral surface is attached to the outer peripheral surface of core spindle member 1 so that the outer peripheral surface of encircling annulus member 2 can cooperate with the inner peripheral surface of sleeve 3 to form a radial bearing. Furthermore, encircling annulus member 2 is provided with thrust dynamic pressure creating grooves 11 on its upper and lower surfaces such that thrust bearings are formed between the lower surface of encircling annulus member 2 and the bottom surface of sleeve 3 and further between the upper surface of encircling annulus member 2 and the underside of cover member 4. Lubricant that serves as working fluid is filled in those parts including at least the radial bearing and the thrust bearings, namely, between the outer peripheral surface of encircling annulus member 2 and the inner peripheral surface of sleeve 3, between the lower surface of encircling annulus member 2 and the bottom surface of sleeve 3, and between the upper surface of encircling annulus member 2 and the underside of cover member 4. This configuration makes it possible to increase the diameter of the radial bearing and hence to achieve improvement in the rigidity of the radial bearing. Additionally, communication hole 12 for allowing the lubricant to flow therethrough is formed between core spindle member 1 and encircling annulus member 2 and further inside encircling annulus member 2. Communication hole 12 acts to compensate the pressure differential which may occur between the top and bottom ends of the radial bearing and further between the inner and outer edges of the thrust bearings due to the cutting errors of the dynamic pressure creating grooves formed in the portions of the radial bearing and the thrust bearings or the cutting errors of the respective components or other factors. Such compensation of the pressure differential helps to suppress bubble generation and excessive rotor floating which would otherwise take place by the negative pressure in the lubricant.
Japanese Patent Laid-open Publication No. 2004-135419 (“prior art reference 2”) teaches a spindle motor arrangement wherein, as illustrated in FIG. 5, encircling annulus member 2 having radial dynamic pressure creating grooves 10 on its outer peripheral surface is attached to the outer peripheral surface of core spindle member 1 so that the outer peripheral surface of encircling annulus member 2 can cooperate with the inner peripheral surface of sleeve 3 to form a radial bearing. Furthermore, sleeve 3 is provided with thrust dynamic pressure creating grooves 11 on its top surface such that a thrust bearing is formed between the top surface of sleeve 3 and the underside of rotor hub 5. Lubricant that serves as working fluid is filled in those spatial parts including at least the radial bearing and the thrust bearings, namely, between the outer peripheral surface of encircling annulus member 2 and the inner peripheral surface of sleeve 3 and between the top surface of sleeve 3 and the underside of rotor hub 5. This configuration makes it possible to increase the diameter of the radial bearing and hence to achieve improvement in the rigidity of the radial bearing. Additionally, communication hole 12 for allowing the lubricant to flow therethrough is formed between the outer peripheral surface of core spindle member 1 and the inner peripheral surface of encircling annulus member 2. Communication hole 12 acts to compensate the pressure differential which may occur in the lubricant retained at the axial top and bottom end portions between the outer peripheral surface of encircling annulus member 2 and the inner peripheral surface of sleeve 3 due to the cutting errors of the dynamic pressure creating grooves formed in the portions of the radial bearing or the cutting errors of the respective components or other factors. Such compensation of the pressure differential helps to suppress bubble generation and excessive rotor floating which would otherwise take place by the negative pressure in the lubricant.
The spindle motor arrangements disclosed in prior art references 1 and 2 however suffer reduction in the diameter of core spindle member 1, which problem stems from the structure that encircling annulus member 2 is attached to the outer peripheral surface of core spindle member 1. This may weaken the coupling force between rotor hub 5 and core spindle member 1, thus reducing the rigidity of the spindle motor as a whole.
With the spindle motor arrangement taught in prior art reference 1, communication hole 12 formed between core spindle member 1 and encircling annulus member 2 is elongated rectilinearly toward bearing openings. Thus, if bubbles are generated by increased dimensional errors or external disturbances such as a shock and the like, the bubbles tend to move up at an accelerated speed as they are discharged to the outside through communication hole 12. This leads to an increased possibility that the lubricant is leaked together in the bubble discharging process.
According to the spindle motor arrangement proposed in prior art reference 2, no communication hole is formed in the portions of the thrust bearing. This means that no means is available to compensate the pressure differential occurring between the inner and outer peripheral surfaces of the thrust bearing due to the cutting errors of thrust dynamic pressure creating grooves 11 and the respective components or other factors. Accordingly, it is impossible to suppress bubble generation and excessive rotor floating caused by the negative pressure in the lubricant. Furthermore, in the event of bubbles being generated by increased dimensional errors or external disturbances such as a shock and the like, the bubbles are blocked off by the dynamic pressure of the thrust bearing and therefore cannot be discharged to the outside. This is because the communication hole has its openings at the radial inner side of the thrust bearing. In addition, due to the fact that the thrust dynamic pressure creating grooves 11 are formed on the top surface of sleeve 3 whose area is quite small, difficulties may be encountered in forming a shoulder portion of desired profile on the top surface of sleeve 3 and in cutting thrust dynamic pressure creating grooves 11 with an enhanced degree of precision. For the same reason, thrust dynamic pressure creating grooves 11 cannot be formed in a cost-effective manner, e.g., through the use of a press-forming method and so forth, thus making it difficult to curtail the manufacturing costs.